api: make EventLoopProxy wrap an opaque type

The proxy is intended to be Clone, thus use `Arc` for it internally and
don't require backends for it to be `Clone`. Use `EventLoopProxyProvider`
to hide the backend's proxy implementation details.

Co-authored-by: daxpedda <daxpedda@gmail.com>

src/platform_impl/web/event_loop/mod.rs

@@ -10,7 +10,6 @@ pub(crate) mod runner;
 mod state;
 mod window_target;
 
-pub(crate) use proxy::EventLoopProxy;
 pub(crate) use window_target::{ActiveEventLoop, OwnedDisplayHandle};
 
 pub struct EventLoop {